The present invention relates to a compact, highly flexible and efficient, multi-positionable, multi-dimensional and multi-stage gas-fired heat exchanger system for use in a forced air duct. The heat exchanger utilizes a cascaded array of serpentine heat transfer tubes. Particularly, the heat exchanger of the present invention utilizes a controller which controls the exhaust fan capacity to ensure a constant supply of gas/air mixture in the tubes which are in operation. The heat exchanger system is also adaptable to existing duct regardless of the size and orientation of the duct.
It is very difficult to substitute a gas heater for an electric hot water or oil-fired heater, as this requires a restructure of the existing ductwork. Accordingly, these conversions are extremely costly and not very practical and popular. However, there exists a commercial need to convert to a gas heater which is highly efficient in terms of energy delivered and non-polluting.
There is on the market place a multitude of gas heaters but these are all of substantially standard dimensions and installed in or equipped with air convection ducts. There is no gas-fired heating equipment on the market today that can be used to economically and efficiently convert an electric heating system to gas. The problem is that the heating equipment are of fixed dimensions, cumbersome and require large installation space. The installation of the equipment is also difficult as it must always be installed horizontal and this requires extensive modification to the ventilation ducts. Also, existing gas heaters need to be mounted horizontal as electric equipment does not. Another problem with gas heaters is that the gas supply as well as the exhaust systems are very cumbersome and do not provide much flexibility to the installer.
An example of a gas-fired heater used in a forced air system is illustrated and described in U.S. Pat. No. 5,368,010. It is a fixed system and it is located in a furnace unit which is supported on a floor located at the base of the ductwork. This patent deals primarily with the evacuation of combustion gases at the outlet of the heat exchange tubes. Reference is also made to U.S. Pat. Nos. 5,042,453, 5,094,224 and 4,729,207 as other examples of gas-fired heaters used as a furnace associated with an air convection duct system.
With multi-stage gas-fired heat exchanger systems, a major problem has been the efficiency of the burners and hence the production of NOx only. In an attempt to resolve NOx reduction, U.S. Pat. No. 5,649,529 teaches disposing a metal mesh tube having a diameter substantially less than the internal diameter of the combustion tubes, at the inlet of the tubes. When the burners operate, the burner flames are forced through the meshtubes to reduce the cross-section of the flames to increase their axial velocity through the associated tube to diminish the intimate contact of secondary combustion air with the maximum temperature zones of the flames within the combustor tube. This arrangement has not proven adequate to resolve the problem, is more costly, and requires additional maintenance and repairs.
With a multi-stage system, there is a constant variation of the number of burners that are operated depending on temperature variation in the space being heated. Therefore, most often, not all tubes are used at the same time. This means that some tubes are often not operational. Therefore, combustion air is drawn through the tubes of least resistance, those which are not in use, and this results in less combustion air being fed to the tubes where the burners are operated and starving them from combustion air thus causing improper combustion and pollution.
We have found a solution to the above problem by modulating the exhaust fan capacity. Because the viscosity of hotter gas is more than cold gas, the tubes that are operational and therefore hotter, provide more resistance to the flow of gases. Therefore, contrary to conventional methods we have found that by increasing fan capacity when less tubes are operational, there is provided an adequate supply of combustion air regardless of the number of tubes in operation. When increasing numbers of tubes are made operational, based on external signal demand (i.e. thermostat), exhaust fan capacity is reduced by the controller and increased when the number of operational tubes are decreased.
It is a feature of the present invention to provide a compact, highly flexible and efficient, multi-positionable, multi-dimensional and multi-stage gas-fired heat exchanger system which is adaptable to existing forced air ducts, and which substantially overcomes the above-mentioned disadvantages of the prior art.
Another feature of the present invention is to provide a gas-fired heat exchanger system as above-described which is easy to install in existing air ducts regardless of the size of the duct and the angular position thereof.
Another feature of the present invention is to provide a gas-fired heat exchanger system as above described and which can be used to readily replace existing electric heaters which are more costly to operate.
Another feature of the present invention is to provide a gas-fired heat exchanger system as above described and which is constructed in accordance with parameters of existing air ducts and wherein the physical characteristics of the construction can be determined by way of a dedicated computer software.
Another feature of the present invention is to provide a gas-fired heat exchanger system as above described comprising a plurality of gas heaters and wherein the gas heaters can be modulated to adjust the heating capacity from about 5% to 100%.
Another feature of the present invention is to provide a gas-fired heat exchanger system as above described and wherein the main component parts of the system are all accessible on a support panel which is located exteriorly of the convection ducts and easily accessible.
Another feature of the present invention is to provide a gas-fired heat exchanger system as above described and wherein a novel turbulator is used within the heat transfer tubes to increase the efficiency of the heat exchanger.
Another feature of the present invention is to provide a gas-fired heat exchanger system as above described and wherein the flue combustion gases can be evacuated regardless of the position of the heat exchanger and by simple means and wherein the exhaust fan capacity is modulated by control means whereby the exhaust fan capacity is controlled in an inversed proportional manner to the number of gas burners in operation to ensure a constant supply of gas/air mixture in the tubes in operation whereby to obtain optimal combustion and maximum efficiency.
According to another feature of the present invention the control means can be effectuated by controlling the speed of the exhaust fan or by any other means of controlling the air supply capacity to the individual tubes such as inlet or outlet air dampers, etc.
According to the above features, from a broad aspect the present invention provides a compact, highly flexible and efficient, multi-positionable and multi-dimensional gas-fired multi-stage heat exchanger system for use in a forced air duct. The heat exchanger comprises a plurality of heat transfer tubes which are provided in numbers depending on the desired BTU/h capacity needs of the heat exchanger. The tubes are secured to a support means. A plurality of gas burners are provided and each mounted on the support means and disposed for directing a flame at an inlet opening of an associated one of the heat transfer tubes. A gas distribution manifold is provided for supplying gas to the burners. A position orientable modulating gas valve is secured to the manifold and connectable to a gas supply line for controlling the gas pressure to the burners and therefore the intensity of the flame. The gas valve is disposed horizontal regardless of the angular position of the system when secured to a duct. A plurality of solenoid valves is secured to the manifold and to a respective one of the burners whereby to operate the burners independently from one another. The tubes each have an outlet connected to a combustion products collector housing having an exhaust fan to evacuate the combustion products. A system programmed controller means is connected to an external heating demand device for receiving an input signal dependent on an air temperature requirement. The solenoid valves are controlled by the controller means dependent on the input signal to control the operation of the gas burners. The controller means controls exhaust fan capacity in an inversely proportional manner to the number of gas burners in operation to ensure a constant supply of gas/air mixture in the tubes in operation whereby to obtain optimal combustion and maximum efficiency.